The present invention relates to an imaging assembly which comprises at least one imaging layer comprising photohardenable microencapsulated coloring agents. Development is accomplished by the application of uniform pressure to the imaging assembly. Improved performance is obtained with respect to the imaging assembly""s response to pressure by preconditioning and sealing the imaging layer to a preselected relative humidity.
The present invention relates to an imaging medium in the form of a self-contained imaging assembly and, more particularly, to an improved self-contained imaging assembly containing a photosensitive imaging layer or layers comprising photohardenable microcapsules encapsulating a coloring material and, outside the microcapsules, a developer material disposed between a first transparent support and a second support which may be opaque or transparent. The imaging medium or assembly can also be referred to as a recording medium, and the imaging layer can be referred to as a recording layer, since the assembly can serve both to capture an image (either the original image or an electronic copy), as does film, and also to display the image, as does a print. Consistent with this fact, the imaging assembly can form a positive image.
The photosensitive imaging layer (including microcapsules) is colored by pressure development after exposure to radiation based on image information. The microcapsules, whose mechanical strength changes (increases) when exposed to light, are ruptured by means of pressure development, whereupon the coloring material and other substances encapsulated in the microcapsules flow out (to varying amounts based on the exposure) and development occurs. The coloring material, such as a substantially colorless color former, migrates to, and reacts with, the developer material and coloring occurs, whereupon a color image is developed. The xe2x80x9crupturexe2x80x9d of the microcapsules are not an all-or-nothing event. Rather, the microcapsules exposed to light are differentially photocured to release varying amounts of color former in order to achieve tonal depth in the exposed area. The differential exposure to light proportionately increases the viscosity of the photocurable composition and thus immobilizes the color former proportionately to the desired tonal depth in the exposed area. The rupture of the microcapsules and the release of the color former is accomplished by the uniform application of pressure. Development of the photosensitive imaging layer can be accomplished, for example, by passing the imaging assembly between a pair of upper and lower nip rollers.
Photohardenable imaging systems employing microencapsulated photosensitive compositions are the subject of various patents, including U.S. Pat. Nos. 4,399,209, 4,416,966, 4,440,846, 4,766,050, 5,783,353, and 5,916,727. Image forming devices (also referred to as printers) are disclosed, for example, in U.S. Pat. No. 4,740,809, wherein exposure occurs by guiding a light from a light source for a plurality of colors across a photosensitive recording medium. U.S. Pat. No. 4,992,822 discloses an image forming device, capable of producing a plurality of colors via a polygonal mirror, for repeatedly exposing the same pixels in a photosensitive recording medium. U.S. Pat. No. 5,893,662 discloses a device for printing an image wherein the device can be incorporated into a computer bay. U.S. Pat. No. 4,648,699 describes a development technique which employs, instead of a pair of nip rollers, a point contact ball moving relative to the photosensitive recording medium.
In the most typical embodiments, the photohardenable composition is a photopolymerizable composition including a polyethylenically unsaturated compound and a photoinitiator and is encapsulated together with a color former. Exposure to actinic radiation hardens the internal phase of the microcapsules. Then, as mentioned above, following exposure, the imaging media in the form of a sheet can be subjected to a uniform rupturing force by passing the sheet through the nip between a pair of pressure rollers.
One of the problems in providing a self-contained imaging assembly that provides a high quality print is image stability or xe2x80x9ckeepingxe2x80x9d which is affected by humidity sensitivity. It is known that print quality, and in particular sensitometric response to actinic radiation, can be significantly affected by sensitivity to humidity and the relative humidity of the environment. Even if the media is manufactured and packaged at a particular humidity, which is found optimum for print quality, variations after the media acclimates to a different later environment can adversely affect the sensitometric properties. This has been believed due to the materials employed in the imaging media, in particular the degree of hardening or curing of the internal phase of a microcapsule and the consequent increase in the viscosity varying with a change in humidity. As a result thereof, photographic characteristics such as speed, maximum density and fogging density (Dmin) are changed from the original optimum. Furthermore, a full color imaging is adversely affected.
In forming a full color image, color precursors which develop into yellow, magenta and cyan colors and photo-initiators corresponding to blue, green and red lights are encapsulated in an internal phase of the microcapsules, and the three sets of the microcapsules are mixed to prepare a full color imaging material containing a developer. The photographic characteristics of the respective microcapsules vary with a change in humidity to different degrees, resulting in muddy colors or incorrect or suboptimal colors. For example, when it is desired that a yellow color be developed, cyan and magenta capsules are cured by red and green lights, and only a yellow color former reacts with a color-developer to form an image. However, if the cyan or magenta capsules insufficiently cure due to a change in humidity, the result may be a muddy color in which cyan or magenta is blended with yellow to some extent. Such muddy colors or other sensitometric phenomena due to a change in humidity has been a significant problem.
One technique that has been used to address the humidity problem and to improve media stability resides is conditioning the layer containing the developer and microcapsules to a relative humidity (RH) of about 10 to 40% and preferably about 20%. For example, U.S. Pat. Nos. 5,916,727 and 5,783,353 disclose conditioning the layer at about 20% RH for about 2 to 12 hours or more, at ambient temperatures, and subsequently sealing the assembly at this low RH level to assure that the layer is relatively moisture-free during the normal shelf-life of the assembly. Furthermore, U.S. Pat. No. 5,783,353 describes that the peripheral edges of the self contained imaging assembly may be sealed additionally, and that sealing of the assembly at low RH levels after conditioning assures that the layer is relatively moisture free during the normal shelf life of the assembly.
U.S. Pat. No. 5,996,793 discloses storing the image-forming material together with a humidity-controlling material. Further, the patent discloses storing the image-forming material and the humidity-controlling material within a package made from a low-moisture permeable film. The low-moisture permeable film can be a plastic film on which is deposited a metal. Other low-moisture permeable films mentioned include fluorinated resins such as polytetrachloroethylene, polytrifluoroethylene, chlorinated rubber, polyvinylidene chloride, a copolymer of polyvinylidene chloride and acrylonitrile, polyethylene, polypropylene, polyesters, and films obtained by depositing a metal such as aluminum and a metal oxide such as silicon oxide.
Unfortunately, when the imaging media are not used right away, especially if a plurality of media are stored for some time in a printing device prior to forming an image, the media may have an opportunity to adjust to ambient humidity and, especially in very dry or very humid climates, the RH of the media may decrease or increase substantially in a short time. Once the imaging media is removed from a package, it does not take very long for the environmental humidity to affect the media. Ambient humidity can soon penetrate the outside surface support on each side of the media causing a change in the moisture content within the media.
After extensive investigation, Applicants have found that humidity affects the mechanical properties of the imaging layers, as compared to the reaction properties of materials during photohardening of the imaging layer. The affect on mechanical properties cause undesirable variations in the degree of rupture of the microcapsules when the media is subjected to pressure during development. Although not wishing to be bound by theory, this may be due to the humidity changing the break strength of the capsules and/or it may be due to the change in pressure applied on the capsules due to the change in the stiffness of the image layer which in turn increases the stress applied to the microcapsules, more likely the latter.
An object of the present invention is, therefore, to provide a self-contained photohardenable imaging assembly that is resistant to the affect of humidity and which will print consistently in response to a means for applying pressure to the assembly.
It would be desirable to obtain an improved media that has no significant change in sensitometric properties with relative humidity, based on speed, Dmax, Dmin, tonal scale, and fall color correctness.
It would be particularly desirable if these objectives could be accomplished without requiring radical changes in conventional imaging chemistry, with respect to the microcapsules and the developer. It would be advantageous if these objectives could be attained in a product that was economical to manufacture and inexpensive for the customer to purchase.
In the self-contained imaging system of the present invention, an imaging layer containing developer and photohardenable microcapsules is placed between two support members to form an integral unit, wherein the imaging layer has been equilibriated within a preselected relative humidity prior to being enclosed between the two support members. In particular, the imaging layer should be equilibriated to a relative humidity of at least 50%. Preferably, the supports include a moisture barrier layer and the imaging assembly is sealed on the sides or edges.
The term xe2x80x9csealed,xe2x80x9d as used herein, refers to a seal which is designed to be non-temporary. This seal is maintained during printing of the image and in the final imaged product, as compared to a temporary package.
In the imaging assembly of the invention, a first support is transparent and a second support may be transparent or opaque. Suitably, the transparent support has a percentage light transmission of at least about 80 percent at a wavelength of 550 nm. In the case of an opaque back support, an image is provided against a substantially white background as viewed through the transparent support and, in the former case, the image is viewed as a transparency preferably using an overhead or slide projector. Sometimes herein the first support may be referred to as the xe2x80x9cfrontxe2x80x9d support and the second support may be referred to as the xe2x80x9cbackxe2x80x9d support.